Synthetic polymer fiber is used in textile fabrics, and for other purposes. For textile fabrics, there are essentially two main fiber categories, namely continuous filament yarns and staple fiber, i.e. cut fiber. Large amounts of filaments are used in small bundles of filaments, without cutting, i.e. as continuous filament yarn, e.g. in hosiery, lingerie and many silk-like fabrics based on continuous filament yarns; the present invention is not concerned with these continuous filament yarns, but with staple fiber and its precursor tow, which are prepared by very different equipment, and which require entirely different handling considerations because of the large numbers of filaments that are handled. Staple fiber has been made by melt-spinning synthetic polymer into filaments, collecting very large numbers of these filaments into a tow, which usually contains many thousands of filaments and is generally of the order of several hundred thousand in total denier, and then subjecting the continuous tow to a drawing operation between a set of feed rolls and a set of draw rolls (operating at a higher speed) to increase the orientation in the filaments, sometimes with an annealing operation to increase the crystallinity, and often followed by crimping the filaments, before converting the tow to staple fiber, e.g. in a staple cutter. One of the advantages of staple fibers is that they are readily blended, particularly with natural fibers, such as cotton (often referred to as short staple) and/or with other synthetic fibers, to achieve the advantages derivable from blending, and this blending may occur before the staple cutter, or at another stage, depending on process convenience.
It has been particularly desirable to blend synthetic staple fiber with cotton, particularly to improve the durability and economics of the fabrics made from the blends with cotton, because such synthetic staple fibers have a high load-bearing tenacity.
Synthetic polyester fibers have been known and used commercially for several decades, having been first suggested by W. H. Carothers, U.S. Pat. No. 2,071,251, and then by Whinfield and Dickson, U.S. Pat. No. 2,465,319. Most of the polyester polymer that has been manufactured and used commercially has been poly(ethylene terephthalate), sometimes referred to as 2G-T. This polymer is often referred to as homopolymer. Commercial homopolymer is notoriously difficult to dye. Such homopolymer is mostly dyed with disperse dyestuffs at high temperatures under elevated pressures, which is a relatively expensive and inconvenient process (in contrast to processes for dyeing several other commercial fibers at atmospheric pressure, e.g. at the boil), and so there have been several suggestions for improving the dyeability of polyester yarns. For instance, Griffing and Remington, U.S. Pat. No. 3,018,272, suggested the use of cationic-dyeable polyesters. Such polyesters, consisting essentially of poly [ethylene terephthalate/5-(sodium sulfo) isophthalate] containing about 2 mol % isophthalate groups in the polymer chain (2G-T/SSI), have been used commercially as a basis for polyester yarns for some 20 years.
Although such polyester fibers have been very useful, it has long been desirable to provide alternative fibers, having the desirable characteristics of commercial polyester fibers accompanied by excellent dyeing properties.
Watson, in U.S. Pat. No. 3,385,831, suggested textile fibers of copolymers of polyethylene terephthalate/hexahydroterephthalate. These fibers showed a surprising combination of enhanced dyeability and good overall physical properties, including low shrinkage values. These copolymer fibers are rather unique, considering the unusually large molar amounts of comonomer (i.e. the hexahydroterephthalate units, HT) in comparison with other comonomers in polymers with ethylene terephthalate (2G-T). Despite the advantages on paper, however, Watson's fibers were not produced in commercial quantities. Some reasons are believed to be the relatively low strength and relatively high sensitivity to elevated temperatures of Watson's fibers. As indicated, several properties do get less desirable as the proportion of comonomer is increased, although the dyeability is correspondingly improved. The improved dyeability from higher proportions of HT comonomer would have been very desirable, if such problems could have been solved.
An object of the present invention is to improve the properties of Watson's type of fibers of copolymers containing ethylene terephthalate (2G-T) and ethylene hexahydroterephthalate (2G-HT) units.